Selective complementation of defective influenza A virus strains and ultimately "rescue" of cloned mutant influenza DNA into influenza virions may require the expression of cloned genes in persistently infected or stably transformed cells. For example, one obstacle to achieving complementation and rescue during lytic infection by our established SV40 vectors is interference of co-infecting influenza virus by replicating SV40. Bovine papilloma virus (BPV) is a large DNA virus that replicates autonomously and extra-chromosomally during persistent infection of animal cells. We recently initiated a collaborative effort with Drs. Peter Howley and Ming-fan Law (NCI) in an effort to exploit their BPV vector for the expression of cloned influenza viral genes. We constructed a BPV recombinant DNA incorporating the influenza nucleoprotein (NP) gene and transformed a mouse cell line (C127) for the persistent expression of NP.